Large electric generators are composed of windings that are capped off with flux traps (end shields) on either end. The flux traps are composed of many layers of insulation coated metal plates that are tightly stacked together, and usually bolted and/or bonded. Along the flux trap perimeter, hollows are present to allow for the through-bolts which provide the main compression for the stator laminations by tightly holding the opposing end plates together.
FIGS. 1A and 1B illustrate this section of the generator. The flux trap 2 is composed of a series of stacked laminations 4 which in this example are bolted together 6, as well as being bolted to the end plate 14 of the generator core. Pockets 8 along the perimeter of the flux trap 2 make room for through-bolts 12 which bind and compress the end plates 14. The entire flux trap is then covered with a support cone (not shown) which is composed of non-conducing materials, such as fiberglass.
During the operation of a generator, the individual laminations 4 of the flux trap have been known to migrate. As they migrate they can come into contact with the through-bolt 6, including the through-bolt nut and through-bolt washer. If this occurs at both ends of a generator core, then a short is created and the generator gets severely damaged, including the melting of the through-bolt and surrounding flux trap and end plate.
Attempts have been made in the prior art to better secure the laminations of the flux trap together to prevent migration. Unfortunately migration can continues to occur and generators become damaged and need to be shut down and repaired. What is needed is a method and apparatus that prevents damage to the generator from lamination migration. What is also further needed is a method and apparatus of retrofitting existing generators with the preventative measure.
Other difficulties with the prior art also exist, some of which will be apparent upon further reading.